我在实现构建器设计模式时遇到了一个问题。

Question

我从一个在线视频教程中学习Java的设计模式。我已经学习了构建器设计模式，我正在尝试实现它所做的事情，但对我来说，它显示了一些错误。帮我解决这些问题。

public class Phone {

    private String model;
    private String os;
    private int ram;
    private double screensize;
    private int battery;
    
    public Phone(String model, String os, int ram, double screensize, int battery) {
        super();
        this.model = model;
        this.os = os;
        this.ram = ram;
        this.screensize = screensize;
        this.battery = battery;
    }
    
    @Override
    public String toString() {
        return "Phone [model=" + model + ", os=" + os + ", ram=" + ram + ", screensize=" + screensize + ", battery="
                + battery + "]";
    }           
}

public class Phonebuilder {

    private String model;
    private String os;
    private int ram;
    private double screensize;
    private int battery;
    
    
    public Phonebuilder setModel(String model) {
        this.model = model;
        return this;
    }
    public Phonebuilder setOs(String os) {
        this.os = os;
        return this;
    }
    public Phonebuilder setRam(int ram) {
        this.ram = ram;
        return this;
    }
    public Phonebuilder setScreensize(double screensize) {
        this.screensize = screensize;
        return this;
    }
    public Phonebuilder setBattery(int battery) {
        this.battery = battery;
        return this;
    }
    
    public Phone getphone () {
        return new Phone(model, os, ram, screensize, battery);
    }
}

public class Test {

    public static void main(String[] args) {
        //It showing error in the below line like type mismatch.

        Phone p = new Phonebuilder().setBattery(9000).setModel("M31").setOs("Android");
        System.out.println(p);
    }

}

Answer 1

您缺少了构建对象的方法调用(getphone())

Phone p = new Phonebuilder()
    .setBattery(9000)
    .setModel("M31")
    .setOs("Android")
    .getphone(); //This builds and returns a Phone instance

Sidenote: IMO buildPhone可能是个更好的名字。

Answer 2

最后一个被调用的setOs方法返回Phonebuilder，但是您将它分配给Phone。我想目的是在设置所有参数之后添加getPhone。

Answer 3

我不认为在使用构建器模式时在Phone类中提供公共构造函数是个好主意。使用构建器模式的原因是为了解决与大量可选参数相关的问题。下面是您的示例的替代方法(它基于图书“有效Java”中的第2项'当面对许多构造函数参数时，请考虑构建器 )。这种方法的一个优点是，电话类是不可变的，即一旦创建，它就不能更改。

  public class Phone {
    private final String model;
    private final String os;
    private final int ram;
    private final double screensize;
    private final int battery;

    private Phone(Builder builder) {
      model = builder.model;
      os = builder.os;
      ram = builder.ram;
      screensize = builder.screensize;
      battery = builder.battery;
    }

    public static class Builder {

      private String model;
      private String os;
      private int ram;
      private double screensize;
      private int battery;

      public Builder calories(String val) {
        model = val;
        return this;
      }

      public Builder os(String val) {
        os = val;
        return this;
      }

      public Builder ram(int val) {
        ram = val;
        return this;
      }

      public Builder screensize(double val) {
        screensize = val;
        return this;
      }

      public Builder battery(int val) {
        battery = val;
        return this;
      }

      public Phone build() {
        return new Phone(this);
      }
    }
  }

创建电话生成器作为-

Phone phone =
        new Phone.Builder()
            .battery(1)
            .calories("100")
            .os("windows")
            .screensize(15)
            .battery(2)
            .build();